To investigate polymer chain morphology under coil-stretch transition, the Weissenberg number \(Wi\) , which is a dimensionless number representing the ratio of elasticity and viscosity, is often used as an indicator.
However, evidence that \(Wi\)  is in fact an indicator of coil-stretch transition is limited to the results of numerical calculations[1]}, [2]}, which depend on the type of constitutive model chosen [3]}, [4]}, [5]}, [6]}, [7]}.
On the other hand, there have been few reports of experimental investigations of the changes in molecular structure of polymer chains under extensional conditions.
Lodge[8]}, [9]} and Philippoff[10]} proposed to measure the flow-induced birefringence of highly viscoelastic solutions reeled in by a draw wheel.
Rothstein and McKinley[11]} developed a method for combined measurements by a filament stretching rheometer and a birefringence measurement system for dilute solutions undergoing extension.
These techniques enable investigation of the effect of extensional stress on the flow-induced birefringence of liquid polymers.
Nevertheless, to demonstrate the validity of the stress-optic law[12]}, [13]}, [14]}, [15]} for a flowing polymer fluid, it is important to simultaneously measure the birefringence and the orientational state of the polymer chain under extensional stress loading.
For this purpose, it is necessary to establish a simple approach to the investigation of the above-mentioned complex problems through birefringence measurements and develop a simple rheo-optical measurement method using a high-speed polarization camera.
